This invention relates generally to mechanism control, particularly this invention relates to control systems of the stored program type known as programmable controllers.
Programmable controllers are known in which the stored program corresponds to a diagram relating switch contacts and relay or actuator coils. The diagrams are structured as rungs in a ladder where elements defining the rungs can be a combination of contacts connected in series and/or parallel ultimately terminating in a single coil. The controller processes the stored program by sequentially advancing through the elements of a rung accumulating the logical result of the current condition of the contacts and setting the coil to an energized or deenergized condition according to the accumulated logical result. Each rung is processed in turn and when all rungs have been processed, the cycle is repeated. The current contact status, i.e., whether the contact is opened or closed, may be determined at the moment a contact is being examined or before or after the ladder diagram processing has been completed. While this ladder diagram program structure has the advantage of familiarity for control designers it has an inherent disadvantage in that modifications to any part of the program require that all processing be inhibited, and consequently that the operation of the controlled mechanism temporarily be suspended. Furthermore, since the program contacts and coils are typically dedicated to particular interface circuits connected to controlled devices, substitutions of interface circuits for program elements requires that program element designations be changed to reflect the new interface circuit assignment. Thus, the entire program must be modified.
It is therefore an object of the present invention to provide a programmable controller having an instruction set readily adapted to program generation for the process of the controlled mechanism which permits program modifications while program execution continues.
It is a further object of the present invention to provide a programmable controller that executes a program including a plurality of instruction sequences representing mechanism cycles of operation and relating variable names corresponding to control devices and variable values.
It is a still further object of the present invention to provide a programmable controller that executes a program including a plurality of instruction sequences relating variable names and which permits modification of selected instruction sequences while execution of other instruction sequences continues.
It is a still further object of the present invention to provide a programmable controller which permits the assignment of interface circuits associated with mechanism devices to be performed independently of the generation of the program of the controlled process.
Further objects and advantages of the present invention shall become apparent from the following description.